This invention relates generally to surgical joint prostheses and more specifically to implant devices that are intended to replace and repair load bearing surfaces in articular joints of the human body.
Commercially available knee, hip, and other articular joint prosthesis have become increasingly more successful and popular in recent years. The original metal-to-metal or bone-to-metal designs were improved by Charnley through the development of bearing surfaces combining polished metal and polyethylene of high molecular weight. This combination provided a bearing surface exhibiting low coefficient of friction and a low rate of wear. Charnley also improved the fixation of prosthesis by the use of polymethylmethacrylate cement to fix the components of a prothesis in the bone.
These developments led, for example, to the knee joint designs of Gunston and Marmor wherein metal runners are cemented to the femur and polyethylene tracks are cemented to the tibia. A patient provided with this type of knee prosthesis essentially has a resurfaced joint with the ultra-high-molecular-weight polyethylene tibial component serving as the load bearing surface. However, the low elastic modulus of ultra-high-molecular-weight polyethylene may, under certain circumstances, permit deformation of the tibial component under load. Due to this deformation, detrimental shear forces are developed between the plastic component and the cement, and this may cause the mechanical bond to weaken.
Thus, schemes for the attachment of a polyethylene articular surface to a metal base plate were developed to improve both the fixation and load distribution of the prosthesis. Such prior methods include the use of a metal retaining wall around the support base with the polymeric articular surface fitted into the retainer by an interference fit or by a locking pin for example. This method is shown in devices known in the art as the Spherocentric knee and the Murray-Shaw knee. In addition, British Pat. No. 719,308 granted to Balog discloses the use of a metal endoskeleton having divergent arms which hold the molded polymeric articular surface in place.
Although generally successful, the methods employed heretofore have not been without their disadvantages. Retaining walls or endoskeletons, in order to provide sufficient lateral resistance to secure the polymeric surface, must be positioned a substantial height above the support plate. This height, in combination with a support plate thickness sufficient to adequately reinforce the articular surfaces, results in a minimum thickness of 8 to 10 mm. for the load bearing portion of the component. Thus, both of these methods are suitable only for thick and relatively deeply concave articular surface designs. In cases where a thin prosthesis is required, due to anatomical or surgical concerns, such methods are unsuitable.
For instance, the strength of an articulating implant and the possibility of future surgical alternatives are dependent largely upon the amount of supporting bone upon which the prosthesis is mounted. Minimum bone removal ensures a stronger cancellous bone structure for firm support and allows a wider choice of surgical alternatives should further modifications become necessary. Any attempt to reduce the thickness of the support or the height of the retaining portions tends to cancel their utility.
It is therefore a general object of the present invention to provide a new and improved articular joint prosthesis.
Another object of the present invention is to provide a thin joint prosthesis having a polymeric articular surface positively secured and supported by a metal base.
Still another object of the present invention is to provide a tibial surface replacement component of minimum profile and having a polymeric articular surface positively secured to a supporting metal plate.